The present invention relates to the field of differential input circuits, and in particular to a differential input circuit for a digital signal processing system that includes an analog difference input and an analog/digital converter, as well as a device for adjusting the DC component in a digitally converted input signal.
Digital signal processing systems are widespread. As known, these systems are technically and economically superior to analog circuits in numerous ways, and are replacing analog circuits in many applications.
In many cases signals are transmitted as analog signals due to the limited bandwidth available in the transmission medium. Consequently, an analog/digital converter (A/D converter) must be connected before the digital signal processing system. To improve noise immunity, input circuits with a difference input are frequently used for this purpose, with the input signal frequently being fed into the A/D converter through an anti-aliasing filter.
However, transmitting signals with the usual modulation methods has the consequence that the received signal does not have a DC component (signal component at frequency zero). Since such a DC component is important for several signals, for example video signals with a porch, this must be specially reconstructed by using information transmitted together with the signal.
Differential input circuits for this purpose are known, in which a DC component of the signal can be recovered and adjusted by resistors and capacitors. However, a disadvantage of these circuits is that, if the resistors are mismatched or if there is an offset in a subsequent anti-aliasing filter and/or the A/D converter, the DC component of the digitally converted input signal will be defective (xe2x80x9cclamping errorxe2x80x9d). In case of video signals, this can result in color garbling or, in a black and white picture, it can lead to color tinge.
European Patent Application EP-A-0539259 describes a differential input circuit with an analog-digital converter, and a microprocessor that evaluates the analog input signals converted by the analog-digital converter and performs corrections to attain a design value.
European Patent Application EP 0714169 A1 describes an input circuit with an analog-digital converter and a comparison circuit, which compares the input signal that has been digitally converted by the analog-digital converter to a design value, to adjust the DC component of the analog input signal.
Japanese Patent Application JP 05055918A describes a differential input circuit with an analog difference input for an operational amplifier. An analog-digital converter is connected after the operational amplifier, and it converts the analog output signal of the operational amplifier into a digital value. The digitally converted value is compared with a design value by an evaluation device. To adjust the DC component of the analog input signal, the analog difference input is charged or discharged in the direction toward the design value.
Therefore, there is a need for a differential input circuit that accurately recovers a DC component of a received signal.
It is an object of the invention to create a differential input circuit of the type mentioned in the introduction, so that the errors in the DC component of an analog transmitted and digitally converted input signal can essentially be avoided.
Briefly, according to an aspect of the present invention, a differential input circuit for a digital signal processing system receives a differential input signal and provides an analog input signal indicative thereof. The circuit digitizes the analog input signal and provides a digital input signal indicative thereof, and compares the DC component of the digital input signal with a DC-component design value. The circuit regulates the magnitude of the analog input signal such that the DC-component of the digital input signal is driven towards the DC-component design value.
An advantage of the inventive input circuit is that, by evaluating the digitally converted input signal, errors caused by an anti-aliasing filter and the A/D converter and possibly other components can be compensated.
Further details, features, and advantages of the invention are apparent from the following description of a preferred embodiment in terms of the drawing.